The position of a mobile device (device), such as, for example, a cellular telephone, may be estimated based on information gathered from various systems. One such system may include the Global Positioning System (GPS), which is one example of a satellite positioning system (SPS). SPS systems such as GPS may include a number of space vehicles (SV) orbiting the earth. Another example of a system that may provide a basis for estimating the position of a mobile device is a cellular communication system including a number of base stations to support communications for a number of mobile devices.
A position estimate, which may also be referred to as a position “fix”, for a mobile device may be obtained, for example, based at least in part on distances or ranges from the mobile device to one or more transmitters, and also based at least in part on the locations of the one or more transmitters. Such transmitters may comprise SVs in the case of an SPS and/or terrestrial base stations in the case of a cellular communication system, for example. Ranges to the transmitters may be based on one or more signals transmitted by the transmitters and received at the mobile device, and/or vice versa. The location of the transmitters may be ascertained, in at least some example implementations, based on the identities of the transmitters, which may be ascertained from one or more signals received from the transmitters.
In certain Code Division Multiple Access (CDMA) digital cellular networks, a position location capability may apply Advanced Forward Link Trilateration (AFLT) techniques. In certain example Wideband Code Division Multiple Access (WCDMA) and Long Term Evolution (LTE) networks a position location capability may apply Observed Time Difference Of Arrival (OTDOA) techniques.
LTE OTDOA positioning technology, by way of example, may use Positioning Reference Signals (PRS) to determine (e.g., measure, calculate, estimate, etc.) a Time Difference of Arrival (TOA) for the positioning signals received from neighboring cells to determine an OTDOA. In order to be able to measure the PRS signals from the serving cell and neighbor cells, a mobile device, may send an assistance data request to an OTDOA system server. Such a server may then send certain assistance data, e.g., possible indicating a suite of cells' information (e.g., Base Station Almanac (BSA) and timing information), to the mobile device. In this example, at least a portion of the information provided to the mobile device by way of such example assistance data may help the mobile device to identify which PRS(s) (cells, transceivers, etc.) to attempt to use for TOA measurements. Note, the terms mobile device, user equipment (UE), and mobile station (MS) are used interchangeably herein and unless otherwise specified are intended to cover any type of electronic device that may participate in the example techniques and/or systems provided herein.
In some instances, an OTDOA measurement may comprise a Reference Signal Time Difference (RSTD). An RSTD may, for example, indicate a relative timing difference between two cells (e.g., a reference cell and a neighbour cell), calculated as the smallest time difference between two subframe boundaries received from the two different cells. The PRS signals may be transmitted by their respective cells on the same carrier frequency (herein, referred to intra-frequency). In other instances, the PRS signals may be transmitted on different carrier frequencies (herein, referred to as inter-frequency). Thus, an RSTD measurement may be calculated for both “intra-frequency” PRS signals and “inter-frequency” PRS signals.
Such TOA-based measurements may be related to the geometric distance between an antenna of the mobile device and an antenna of the transmitting base station. In certain instances, however, frequency components of a positioning signal may encounter a time delay (herein, referred to as group delay), e.g., while being processed by circuitry in a device, such as an amplifier, while propagating through a medium such as air, etc. When performing intra-frequency measurements, a group delay of each signal transmitted on the same frequency may be the same and thus, may be easily discounted. However, when performing inter-frequency measurements, accurate positioning may be difficult to achieve without compensating in some manner for the different delays encountered by the differing frequencies of the positioning signals.
Some systems may attempt to provide inter-frequency compensation by including a static look-up table in the mobile device which may include various inter-frequency bias compensation values for the carrier frequencies that a particular mobile device supports. However, the group delays encountered by certain frequencies may vary over time, for example, due in-part to changing environmental factors, such as temperature. Thus, an inaccurate bias compensation may be applied at times, which may reduce the accuracy of such inter-frequency measurements, and possibly the accuracy of a corresponding position fix for the mobile device.